1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to an apparatus and method for controlling the power levels of respective transmitters to achieve respective desired received signal quality levels in the wireless communication systems.
2. Description of the Related Art
In a cellular telephone or personal communication system (PCS), a large number of “mobile stations” communicate through cell sites or “base stations”. Power control (PC) is one of the most important issues in cellular systems. There are three missions for a PC scheme. The first one is to resolve the near-far problem, which is accomplished by “outer-loop”(open-loop) PC. The second one is to mitigate the short-term fading so as to minimize the variation of the received signal power or signal to interference ratio (SIR), which is accomplished by “inner-loop”(closed-loop) PC. The third one is to save transmission power for longer battery life, which is usually related to the setting or adaptation of the threshold used in the PC algorithm. The transmitted signal experiences multipath fading as the mobile station moves in the environment that reflects the signal. Controlling mobile station transmitter power to overcome multipath fading is described in U.S. Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA MOBILE CELLULAR TELEPHONE SYSTEM,” issued on Oct. 8, 1991 and incorporated herein by reference.
If the mobile station transmits an excessively powerful signal, it will interfere with the transmitted signals of other mobile stations. If the mobile station transmits an insufficiently powerful signal, the base station will be unable to recover the transmitted information from the received signal. In the above-referenced patent, the base station measures the power of the signal received from a mobile station and transmits power control commands to the mobile station over a separate channel. The commands instruct the mobile station to increase or decrease transmission power to maintain the average received signal power at a predetermined level. The base station must periodically adjust the transmission power of the mobile station to maintain an acceptable balance between interference and signal quality as the mobile station moves.
Given perfect outer-loop PC, the inner-loop PC schemes can be mainly divided into two categories: the strength-based PC scheme and the SIR-based PC scheme. The inner-loop power control in a strength-based manner is described in U.S. Pat. No. 5,257,283, entitled “SPREAD SPECTRUM TRANSMITTER POWER CONTROL METHOD AND SYSTEM,” issued on Oct. 26, 1993 and in U.S. Pat. No. 5,267,262, entitled “TRANSMITTER POWER CONTROL SYSTEM,” issued on Nov. 30, 1993 and incorporated herein by reference. But they did not mention how to set or adjust the desired received power threshold to achieve the required signal quality.
Method and apparatus to adjust the desired received power threshold is described in U.S. Pat. No. 5,216,692, entitled “METHOD AND APPARATUS FOR ADJUSTING A POWER CONTROL THRESHOLD IN A COMMUNICATION SYSTEMS,” issued on Jun. 1, 1993 and in U.S. Pat. No. 5,396,516, entitled “METHOD AND SYSTEM FOR THE DYNAMIC MODIFICATION OF CONTROL PARAMETERS IN A TRANSMITTER POWER CONTROL SYSTEMS,” issued on Mar. 7, 1995 and incorporated herein by reference. While, the major problem is that the power control threshold is adjusted in a step-wise manner, wherein the side effects include that a large adjustment step size can not fine tune the threshold value, while a small one will increase the response time of the power control mechanism.
Moreover, the SIR value is usually considered as an indicator of the signal quality and the SIR formula is the basic to design a SIR-based PC mechanism. The SIR-based PC is a more direct way to achieve the required signal quality, defined by SIR value, than the strength-based PC. Interestingly, there were many different forms of SIR formula used in the art. For example, the generation and use of SIR formula on PC method in the art with detail is provided in the article “Optimal power control in interference-limited fading wireless channels with outage-probability specifications,” S. Kandukuri and S. Boyd, IEEE Transactions on Wireless Communications, Vol. 1, No. 1, pp. 46–55, January 2002. In their cases, to compute SIR value will need the sum of the individual interference signal power, however, which is impossible to be practically measured.
Accordingly, it would be desirable to calculate the power-level threshold values directly, instead of being adjusted in a step-wise manner, so that each required received signal SIR, which is measurable and can truly reflect the signal quality, is achieved. It would be also desirable to have a SIR-based power control according to such a SIR measurement as another way of implementing power control to achieve the desired signal quality. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.